Oil Sands - SAGD Water Recycle

Oil Sands

Oil sands or, more technically, bituminous sands, are a type of unconventional petroleum deposit. Oil sands represent a vast wealth of energy reserves for the world rivaling traditional sources in quality, availability and the ability to meet the world's needs for many years into the future. The technologies for extraction of this resource are increasingly efficient, and WesTech has worked with several companies on the leading edge of these efforts and is a ready partner for new projects and plant upgrades.

The term “oil sands” is a bit of a misnomer. The deposits are saturated with a tarlike substance known as bitumen. A great deal of processing is required to separate this bitumen from the associated soil and other debris. The most common way to recover bitumen from oil sands is through the use of steam to heat the bitumen. This reduces the viscosity so that it can be pumped to the surface in much the same way as crude oils. Oil sands deposits which lie deeper than 75 meters are recovered in this manner. This method is used to extract approximately 90% of the oil sands deposits in the major regions of Canada.

Steam Assisted Gravity Drainage (SAGD)

Steam assisted gravity drainage (SAGD) is a technique where two horizontal wells are drilled into the deposit. These horizontal wells may extend for miles in all directions. One well is directly above the other. In the upper horizontal well, steam is injected continuously to heat the bitumen. The lower of the two wells is used to pump the bitumen to the surface.

The bitumen produced via SAGD contains a significant amount of water from the steam condensate. Since the quality of the water required for discharge is high and the availability of raw water is limited, the choice is most often made to recycle this water.

Some of the major differences from conventional water treatment systems are the desire to preserve as much heat as possible and the need to reduce the high amounts of silica which are common in water at SAGD operations. Heat recovery saves energy in cold climates. In addition, silica reduction via warm lime softening requires temperatures of 140° F for best silica reduction.

Oil/Water Separator

Water treatment begins with the separation of free oil. This can be done either in a conventional rectangular oil/water separator or in a circular oil/water separator as manufactured by WesTech. Once the free oil has been removed, the effluent flows to a dissolved gas flotation (DGF) unit where the dispersed oil is floated to the top by means of dissolved gas bubbles. Both these units are gas tight to prevent the release of volatile organic compounds (VOC’s) and to preserve as much heat value as possible.

Warm Lime Softener

The effluent of the DGF unit is pumped to a warm lime softener. As the name implies, this unit employs traditional lime softening run at an elevated temperature of 140° F. At this elevated temperature the removal of silica is greatly enhanced. This removal rate can be as high as 80-90%.

In addition, the lime softening reduces any hardness and acts as a final oil removal step to ensure there is no oil contamination of downstream processing. From the warm lime softening, the water is pumped through dual media filters to remove any suspended solids in the lime softening effluent. This water then goes through weak acid cation units for further polishing prior to the steam generators.

Oil Sands

Download the Oil Sands Flow Sheets

Oil Sands - Open Pit

Oil Sands

Oil sands or, more technically, bituminous sands, are a type of unconventional petroleum deposit. Oil sands represent a vast wealth of energy reserves for the world rivaling traditional sources in quality, availability and the ability to meet the world's needs for many years into the future. The technologies for extraction of this resource are increasingly efficient, and WesTech has worked with several companies on the leading edge of these efforts and is a ready partner for new projects and plant upgrades.

Open Pit Oil Sands Mining

The term “oil sands” is actually a bit of a misnomer. The deposits are saturated with a tarlike substance known as bitumen. A great deal of processing is required to separate this bitumen from the associated soil and other debris. One of the two most common ways to recover bitumen from oilsands is through open pit mining of deposits which lie near the surface. Oil sands deposits which lie within 75 meters of the surface are typically recovered via mining. This process is much the same as strip mining for coal or any other mineral.

Mining shovels remove the oil sand and load it into large mining trucks. These trucks carry the oil sands to mobile crushers. The crushed material is stockpiled for the next step.

Slurry

The oil sands broken up in these crushers are then fed to rotary breakers with the addition of hot water to remove rocks and other debris. The resulting slurry is pumped through a pipeline and chemicals are added as required. The slurry reports to a primary separator where it is classified into three distinct cuts – the overflow, the middle means, and the underflow.

The middle means are sent to flotation units where the floating material is recovered and returned to the head of the primary separator. The underflow from the flotation units is combined with the primary separator underflow and sent to a trash screen. The oversized material from the screen is washed and and is returned to the mine via pipeline to fill in mined-out areas. The undersized material is sent to a further bank of flotation units. Floated material off the secondary flotation units is also recovered to the head of the primary separator while the underflow is sent to the tailings thickener.

Bitumen

The overflow from the primary separator is sent for processing via steam heating of the bitumen. Bitumen is deficient in hydrogen. Bitumen must be upgraded to synthetic crude oil specification in order to be an acceptable feedstock for refineries. This is done by the addition of hydrogen or the rejection of carbon, or both. Upgrading uses natural gas as a source of heat and steam for processing and also as a source of hydrogen. Other hydrocarbons such as naphtha may also be used for upgrading.

In the tailings thickeners the suspended solids are settled to a sludge which is sent to a horizontal vacuum belt filter for dewatering. The filtrate from the horizontal belt filter is returned to the head thickener for reprocessing. The dried cake from the horizontal belt filter is sent to tailings piles or landfills for disposal.

Overflow

The overflow of the tailings thickener is water which is recovered for recycling back into the circuit. This is not solely due to restrictions on water usage. It is therefore critical that treatment processes involving water recovery in reuse are employed in this application. The combination of tailings thickener(s) and vacuum dewatering equipment results in maximum water recovery.